Take home interviews should be time-limited and proctored real-time. If it's more than 4-6 hours then it's too long. The problem should have multiple parts and just difficult enough to make it unlikely for most candidates to complete everything. The problem should unique enough and broad enough to allow the candidate to demonstrate good code structure, problem solving, creativity, and algorithmic ability. It should obviously not be something so straightforward and simple as to be a question on stack overflow.

The candidate should be encouraged to submit early and often for feedback, and to not be afraid to ask questions. Sometimes a subtle or complex element of the problem statement can help spur this interaction.

Finally the grading should be a blend of both objective criteria such as a series of heuristics and unit tests, as well as subjective measures such as code cleanliness and structure.

Multiple people should be involved in the grading, and a repository of previous submissions should be compared against for consistency.

I and my team used this technique for years to great success, hiring some of the world's top talent.